Encapsulation as used herein is a process in which a small, discrete unit of particulate matter, commonly referred to as the core, is coated with one or more layers of a coating material in order to prevent premature contact of the core with the surrounding environment. The coating or coatings isolate the core particle for later release under controlled conditions.
Nearly any substance capable of being formed into discrete particles may be encapsulated. Core matrials may consist of single substances or mixtures of several substances; and may be solid, liquid, or gaseous in nature. Core materials are typically those substances or mixtures which, if utilized unencapsulated would be utilized or deactivated before performing an intended function. The following list typifies general classes of materials which have been encapsulated:
______________________________________ adhesives bacteria blowing agents catalysts curing agents detergents drugs dyes flavors foods fuels inks insecticides leavening agents metals monomers coils paints perfumes photographic agents pigments plasticizers propellants solvents stabilizers viruses vitamins. ______________________________________
A wide variety of coating materials have been used to encapsulate core particles. The most commonly used coating materials are natural or synthetic polymers, including, for example, gelatin, ethyl cellulose, or poly(methylmethacrylate). Typical coating materials include:
______________________________________ gelatin gum arabic starches sugars ethyl cellulose carboxymethyl shellac rosin cellulose paraffin tristearin polyethers polyethylene polypropylene polybutadiene polystyrene polyacrylamides epoxies polyesters polyamides aluminum polyisoprene silicones copper polyurethanes silicates silver. ______________________________________
A great deal of attention has been directed to coating compositions used in blended cleaning agents in order to protect each component from the harmful degradation effect of other components. Typically bleaching agents can react with organic cleaning agents during manufacture and storage. Such reactions can reduce the active concentration of both bleach and cleaning agent. In the formulation of detergents it is also difficult to maintain an effective concentration of bleach in the detergent composition. Typical bleach compositions are relatively unstable in the presence of alkaline compounds and free moisture. Known coating compositions simply do not segregate reactive compositions to prevent significant loss of bleaching and cleaning activity.
The numerous materials which are used as ingredients in detergent formulations may be divided into the following groups: (a) surfactants; the major cleansing constituent of detergents; (b) diluents or fillers; inorganic salts, acids, and bases which do not contribute to detergency; (c) builders; additives which enhance the detergency, foaming power, emulsifying power, or soil suspending effect of the composition; and (d) special purpose additives such as (i) bleaching agents, (ii) brightening agents, (iii) bacteriocides, and (iv) emollients.
Many attempts have been made to manufacture a detergent composition containing a stable bleaching component including encapsulation of the bleach. Many different encapsulation methods and coating materials have been used in an attempt to obtain a low cost, efficiently encapsulated bleach. Examples of such attempts are disclosed in Brubacher, U.S. Pat. No. 4,279,764, (chemical encapsulation of a chlorine bleaching agent with a silicate bound, hydrated, soluble salt containing an N-H chlorine accepting component); Hudson, U.S. Pat. No. 3,650,961 (fluidized bed enacapsulation of chloroisocyanurate with an inorganic salt); and Alterman, U.S. Pat. Nos. 3,908,044 and 3,908,045, (double coat fluidized bed encapsulation of a chlorine releasing agent with a first coat of a fatty acid having 12 to 22 carbon atoms and a second coat of a fixed alkali hydroxide).
Current emphasis in the encapsulation art is directed to the encapsulation efficiency of the process. Encapsulation efficiency is typically determined by measuring the percentage of core material released into solution after a specified time period when placed in a dissolving environment. Several of the attempted encapsulation processes have been able to increase the encapsulation efficiency above that typically achieved but reach such results at great expense and/or through a difficult process.
It is a commonly held belief that the low encapsulation efficiency is due in major part to a failure to (i) completely coat the core particle, (ii) uniformly coat, and/or (iii) prevent the development of cracks, pores or fissures in the coating.
One of the major difficulties encountered in achieving inexpensive high encapsulation efficiency is that the coating must be applied in "molten form". Ideally, the coating should be added as a flowable liquid to allow it to flow around the core, sealing the core without gaps and/or cracks. However, if added as a flowable liquid, the coating can often fail to adhere to the core and can leave an insufficiently coated core.
The process temperature at which the coating agent is added has been found to be critical. If the temperature is too low the coating can contain numerous fissures or cracks that can be due to poor wetting of the surface of the core or inability of the coating to adhere to the core and if too high (above the melting point of the coating agent) the coating may cause agglomeration of the particles or cause a complete collapse of the fluidized particles. Encapsulation efficiency is improved when the process temperature is held just below the melting point of the coating agent. Such critical process temperatures result in long batch cycle times and the need for critical temperature control.
In an attempt to achieve the desired viscosity, coating compounds are often mixed with a volatile solvent. While this often increases the coating efficiency, the use of volatile solvents is dangerous as (i) many solvents are flammable and explosive, and (ii) many solvents are toxic if inhaled. In addition the use of solvents is expensive as (i) the costly solvent is typically used in large quantities and must be recovered and (ii) expensive "explosion proof" equipment must be used such as static electricity control systems, explosion vents, reinforced equipment, solvent recovery systems and the like.
Accordingly, a substantial need exists for a simple, inexpensive solventless encapsulation process which works with a wide combination of core and coating compounds and results in a highly efficient encapsulation product.